Transcript Real-time Software Design

Real-time Software Design
Chapter 15 of Ian Sommerville’s Book
on Software Engineering
Objectives
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To explain the concept of a real-time system
and why these systems are usually
implemented as concurrent processes
To describe a design process for real-time
systems
To explain the role of a real-time operating
system
To introduce generic process architectures for
monitoring and control and data acquisition
systems
Topics covered
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System design
Real-time operating systems
Monitoring and control systems
Data acquisition systems
Real-time systems
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Systems which monitor and control their
environment.
Inevitably associated with hardware devices
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Sensors: Collect data from the system
environment;
Actuators: Change (in some way) the system's
environment;
Time is critical. Real-time systems MUST
respond within specified times.
Definition
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A real-time system is a software system where the
correct functioning of the system depends on the
results produced by the system and the time at which
these results are produced.
A soft real-time system is a system whose operation
is degraded if results are not produced according to
the specified timing requirements.
A hard real-time system is a system whose operation
is incorrect if results are not produced according to
the timing specification.
Stimulus/Response Systems
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Given a stimulus, the system must produce a
response within a specified time.
Periodic stimuli. Stimuli which occur at
predictable time intervals
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For example, a temperature sensor may be polled 10 times
per second.
Aperiodic stimuli. Stimuli which occur at
unpredictable times
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For example, a system power failure may trigger an
interrupt which must be processed by the system.
Architectural considerations
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Because of the need to respond to timing demands
made by different stimuli/responses, the system
architecture must allow for fast switching between
stimulus handlers.
Timing demands of different stimuli are different so a
simple sequential loop is not usually adequate.
Real-time systems are therefore usually designed as
cooperating processes with a real-time executive
controlling these processes.
A real-time system model
Sensor/actuator processes
System elements
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Sensor control processes
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Data processor
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Collect information from sensors. May buffer
information collected in response to a sensor
stimulus.
Carries out processing of collected information
and computes the system response.
Actuator control processes
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Generates control signals for the actuators.
Real-time programming
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Hard-real time systems may have to
programmed in assembly language to ensure
that deadlines are met.
Languages such as C allow efficient
programs to be written but do not have
constructs to support concurrency or shared
resource management.
Java as a real-time language
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Java supports lightweight concurrency (threads and
synchronized methods) and can be used for some
soft real-time systems.
Java 2.0 is not suitable for hard RT programming
but real-time versions of Java are now available that
address problems such as
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Not possible to specify thread execution time;
Different timing in different virtual machines;
Uncontrollable garbage collection;
Not possible to discover queue sizes for shared resources;
Not possible to access system hardware;
Not possible to do space or timing analysis.
System design
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Design both the hardware and the software
associated with system. Partition functions to
either hardware or software.
Design decisions should be made on the
basis on non-functional system requirements.
Hardware delivers better performance but
potentially longer development and less
scope for change.
R-T systems design process
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Identify the stimuli to be processed and the
required responses to these stimuli.
For each stimulus and response, identify the
timing constraints.
Aggregate the stimulus and response
processing into concurrent processes. A
process may be associated with each class
of stimulus and response.
R-T systems design process
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Design algorithms to process each class of
stimulus and response. These must meet the
given timing requirements.
Design a scheduling system which will
ensure that processes are started in time to
meet their deadlines.
Integrate using a real-time operating system.
Timing constraints
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May require extensive simulation and
experiment to ensure that these are met by
the system.
May mean that certain design strategies such
as object-oriented design cannot be used
because of the additional overhead involved.
May mean that low-level programming
language features have to be used for
performance reasons.
Real-time system modelling
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The effect of a stimulus in a real-time system may
trigger a transition from one state to another.
Finite state machines can be used for modelling
real-time systems.
However, FSM models lack structure. Even
simple systems can have a complex model.
The UML includes notations for defining state
machine models
See Chapter 8 for further examples of state
machine models.
Petrol pump state model
Real-time operating systems
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Real-time operating systems are specialised
operating systems which manage the processes in
the RTS.
Responsible for process management and
resource (processor and memory) allocation.
May be based on a standard kernel which
is used unchanged or modified for a particular
application.
Do not normally include facilities such as file
management.
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Operating system components
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Real-time clock
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Interrupt handler
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Chooses the next process to be run.
Resource manager
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Manages aperiodic requests for service.
Scheduler
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Provides information for process scheduling.
Allocates memory and processor resources.
Dispatcher
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Starts process execution.
Non-stop system components
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Configuration manager
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Responsible for the dynamic reconfiguration of the system
software and hardware. Hardware modules may be
replaced and software upgraded without stopping the
systems.
Fault manager
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Responsible for detecting software and hardware faults and
taking appropriate actions (e.g. switching to backup disks)
to ensure that the system continues in operation.
Real-time OS components
Process priority
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The processing of some types of stimuli must
sometimes take priority.
Interrupt level priority. Highest priority which
is
allocated to processes requiring a very fast
response.
Clock level priority. Allocated to periodic
processes.
Within these, further levels of priority may be
assigned.
Interrupt servicing
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Control is transferred automatically to a
pre-determined memory location.
This location contains an instruction to jump to
an interrupt service routine.
Further interrupts are disabled, the interrupt
serviced and control returned to the interrupted
process.
Interrupt service routines MUST be short,
simple and fast.
Periodic process servicing
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In most real-time systems, there will be several
classes of periodic process, each with different
periods (the time between executions),
execution times and deadlines (the time by
which processing must be completed).
The real-time clock ticks periodically and each
tick causes an interrupt which schedules the
process manager for periodic processes.
The process manager selects a process which
is ready for execution.
Process management
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Concerned with managing the set of
concurrent processes.
Periodic processes are executed at prespecified time intervals.
The RTOS uses the real-time clock to
determine when to execute a process taking
into account:
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Process period - time between executions.
Process deadline - the time by which processing
must be complete.
RTE process management
Process switching
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The scheduler chooses the next process to
be executed by the processor. This depends
on a scheduling strategy which may take the
process priority into account.
The resource manager allocates memory and
a processor for the process to be executed.
The dispatcher takes the process from ready
list, loads it onto a processor and starts
execution.
Scheduling strategies
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Non pre-emptive scheduling
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Pre-emptive scheduling
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Once a process has been scheduled for execution, it runs
to completion or until it is blocked for some reason (e.g.
waiting for I/O).
The execution of an executing processes may be stopped if
a higher priority process requires service.
Scheduling algorithms
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Round-robin;
Rate monotonic;
Shortest deadline first.
Monitoring and control systems
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Important class of real-time systems.
Continuously check sensors and take actions
depending on sensor values.
Monitoring systems examine sensors and
report their results.
Control systems take sensor values and
control hardware actuators.
Generic architecture
Burglar alarm system
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A system is required to monitor sensors on
doors and windows to detect the presence of
intruders in a building.
When a sensor indicates a break-in, the
system switches on lights around the area
and calls police automatically.
The system should include provision for
operation without a mains power supply.
Burglar alarm system
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Sensors
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Movement detectors, window sensors, door sensors;
50 window sensors, 30 door sensors and 200 movement
detectors;
Voltage drop sensor.
Actions
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When an intruder is detected, police are called
automatically;
Lights are switched on in rooms with active sensors;
An audible alarm is switched on;
The system switches automatically to backup power when
a voltage drop is detected.
The R-T system design process
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Identify stimuli and associated responses.
Define the timing constraints associated with
each stimulus and response.
Allocate system functions to concurrent
processes.
Design algorithms for stimulus processing and
response generation.
Design a scheduling system which ensures that
processes will always be scheduled to meet
their deadlines.
Stimuli to be processed
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Power failure
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Generated aperiodically by a circuit monitor.
When received, the system must switch to backup
power within 50 ms.
Intruder alarm
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Stimulus generated by system sensors. Response
is to call the police, switch on building lights and
the audible alarm.
Timing requirements
Stimulus/Response
Power fail interrupt
Door alarm
Window alarm
Movement detector
Audible alarm
Lights switch
Communications
Voice synthesiser
Timing requ irements
The switch to backup power must be comp leted
within a deadline of 50 ms .
Eac h door alarm should be polled twice per
second.
Eac h window alarm should be polled twice per
second.
Eac h mo vement detector should be polled twice
per second.
The audible alarm should be switched on within
1/2 second of an alarm being raised by a sensor.
The lights should be switched on within 1/2
second of an alarm b eing raised by a sensor.
The call to the police should be started within 2
seconds of an alarm being raised by a sensor.
A synthesised message should be available
within 4 seconds of an alarm being raised by a
sensor.
Burglar alarm system processes
Building_monitor process 1
class BuildingMonitor extends Thread {
BuildingSensor win, door, move ;
Siren
siren = new Siren () ;
Lights
lights = new Lights () ;
Synthesizer synthesizer = new Synthesizer () ;
DoorSensors doors = new DoorSensors (30) ;
WindowSensors windows = new WindowSensors (50) ;
MovementSensors movements = new MovementSensors (200) ;
PowerMonitor pm = new PowerMonitor () ;
BuildingMonitor()
{
// initialise all the sensors and start the processes
siren.start () ; lights.start () ;
synthesizer.start () ; windows.start () ;
doors.start () ; movements.start () ; pm.start () ;
}
Building monitor process 2
public void run ()
{
int room = 0 ;
while (true)
{
// poll the movement sensors at least twice per second (400 Hz)
move = movements.getVal () ;
// poll the window sensors at least twice/second (100 Hz)
win = windows.getVal () ;
// poll the door sensors at least twice per second (60 Hz)
door = doors.getVal () ;
if (move.sensorVal == 1 | door.sensorVal == 1 | win.sensorVal == 1)
{
// a sensor has indicated an intruder
if (move.sensorVal == 1)
room = move.room ;
if (door.sensorVal == 1)
room = door.room ;
if (win.sensorVal == 1 ) room = win.room ;
}
lights.on (room) ; siren.on () ; synthesizer.on (room) ;
break ;
}
Building_monitor process 3
lights.shutdown () ; siren.shutdown () ; synthesizer.shutdown () ;
windows.shutdown () ; doors.shutdown () ; movements.shutdown () ;
} // run
} //BuildingMonitor
Control systems
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A burglar alarm system is primarily a
monitoring system. It collects data from
sensors but no real-time actuator control.
Control systems are similar but, in response
to sensor values, the system sends control
signals to actuators.
An example of a monitoring and control
system is a system that monitors temperature
and switches heaters on and off.
A temperature control system
Data acquisition systems
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Collect data from sensors for subsequent
processing and analysis.
Data collection processes and processing
processes may have different periods and
deadlines.
Data collection may be faster than processing
e.g. collecting information about an explosion.
Circular or ring buffers are a mechanism for
smoothing speed differences.
Data acquisition architecture
Reactor data collection
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A system collects data from a set of sensors
monitoring the neutron flux from a nuclear
reactor.
Flux data is placed in a ring buffer for later
processing.
The ring buffer is itself implemented as a
concurrent process so that the collection and
processing processes may be synchronized.
Reactor flux monitoring
A ring buffer
Mutual exclusion
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Producer processes collect data and add it to
the buffer. Consumer processes take data from
the buffer and make elements available.
Producer and consumer processes must be
mutually excluded from accessing the same
element.
The buffer must stop producer processes
adding information to a full buffer and consumer
processes trying to take information from an
empty buffer.
Ring buffer implementation 1
class CircularBuffer
{
int bufsize ;
SensorRecord [] store ;
int numberOfEntries = 0 ;
int front = 0, back = 0 ;
CircularBuffer (int n) {
bufsize = n ;
store = new SensorRecord [bufsize] ;
} // CircularBuffer
Ring buffer implementation 2
synchronized void put (SensorRecord rec )
throws InterruptedException
{
if ( numberOfEntries == bufsize)
wait () ;
store [back] = new SensorRecord (rec.sensorId,
rec.sensorVal) ;
back = back + 1 ;
if (back == bufsize)
back = 0 ;
numberOfEntries = numberOfEntries + 1 ;
notify () ;
} // put
Ring buffer implementation 3
synchronized SensorRecord get () throws InterruptedException
{
SensorRecord result = new SensorRecord (-1, -1) ;
if (numberOfEntries == 0)
wait () ;
result = store [front] ;
front = front + 1 ;
if (front == bufsize)
front = 0 ;
numberOfEntries = numberOfEntries - 1 ;
notify () ;
return result ;
} // get
} // CircularBuffer